1. Technical Field
The present invention relates in general to image capture systems and in particular to digital image capture systems. Still more particularly, the present invention relates to digital image capture systems which automatically adjust brightness and contrast to achieve an optimum image reproduction.
2. Background Art
Digital image capture systems are well known in the prior art. Such systems typically include a video scanning device such as a camera and a digital system such as a personal computer. By utilizing these two devices together in conjunction with a suitable interconnection it is possible to capture and store an image within the memory of a computer and thereafter utilize that image in conjunction with presentations or to create additional copies of that image at a later time.
One problem which is associated with such image capture systems is the determination of the brightness and contrast setting which will be utilized to capture a particular image. Each point within an image to be captured has a brightness level which is located somewhere within a grayscale. The darkest point in an image may not reach full black and the brightest point within an image may not reach full white. Known image capture systems translate this brightness at various points throughout the image into a voltage where black and white represent selected points on a continuous voltage scale.
Since known image capture systems generally store the image as a plurality of digital values it is necessary to convert the continuous analog voltage which represents the brightness of the image to a plurality of digital values for each point within the image. Generally, the number of possible digital values which may be assigned to a particular point within an image is a function of the hardware which is utilized to convert the analog signal to a digital signal. These digital numbers must be accurately assigned throughout the range of possible analog voltages in order to obtain an optimum image. For example, digital values should not be assigned to analog voltages which are beyond those which occur in the image. Similarly, if all of the digital values are compressed to the point where they do not accurately represent the full dynamic voltage range of the image some of the detail in the image will not be captured.
Attempts to solve this problem are well known in the art. For example, "electric eye" cameras have been present in the photographic art for a generation. These cameras work by setting the middle of a fixed grayscale index to the average brightness of a particular scene. The limitations of this particular approach are well known. For example, if you aim an automatic video camera at a bride in a white gown against a white altar the resultant image will depict a bride in gray against a gray altar. Similarly, if you aim an automatic video camera at a photograph of fireworks against a black sky, every wrinkle within the photographic paper will be visible but the fireworks will be grossly washed out. Those skilled in the art will appreciate that exposing an image in response to the average brightness of that image works only approximately and only under certain conditions.
In recognition of this short fall several modern photographic camera metering systems have been proposed which utilize a group of multiple sensors, each of which views a different region within a scene or image to be captured. The results of these multiple readings are then utilized as a pointer within a large memory device. Within the large memory device are stored the research results of thousands of different photographic situations. Theoretically, these situations take into account relative differences, absolute image brightness and camera orientation. While this system works much better than a simple average brightness system it is not perfect and may be mislead.
While the foregoing description of the prior art describes these systems in terms of black and white images, those skilled in the art will appreciate that color image systems experience similar problems. Indeed, brightness and contrast for each of the three primary colors must be adjusted correctly in order to obtain a quality color image reproduction. In a color system each of the three primary colors includes a "white level adjustment" and a "black level adjustment" which is utilized to set the voltage limits of the analog voltage representing image brightness to a digital number conversion. These adjustments must be repeated for each of the three primary colors, for a total of six adjustments. In the color image art these adjustments are often referred to in terms of the primary color associated therewith such as "red white level" or "green black level." It is very difficult for even an expert user to handle all six variables and the above-described prior art automatic adjustment systems are very often fooled into incorrect color settings.
In view of the above, it should be apparent that a need has existed for a method and apparatus wherein the brightness and contrast level in an image capture system may be accurately and rapidly determined in order to produce an accurate reproduction of an image.